Using the vacuum method, even minimum leakage rates of a container can be reliably detected. The smaller the leakage rate is, the higher will be the demands posed on cleanliness and end vacuum. When searching for leaks in situ, the container is evacuated by use of a leak detector until the test pressure required for the leak detector has been reached. Then, suspected leak sites on the container are sprayed from outside with a fine test gas jet. Test gas entering the container will be pumped off by the vacuum pump means and be detected by a mass spectrometer. A leak rate measuring device of this type is described in the brochure “Industrielle Dichtheits-Prüfung” of Leybold-Heraus GmbH dated 1987.
The mass spectrometers used in leak rate measurement are strip spectrometers such as e.g. the sector-field mass spectrometer wherein gas ions are caused to follow a curved path and then to pass an opening arranged within a shutter so as to have the gas ions impinge on an ion catcher electrode. The latter is connected to a highly sensitive electrometer amplifier by which the very small stream of ions will be sufficiently amplified for supplying the ions to a follower amplifier. An anode is fed with a mass-specific anode voltage. This voltage will cause a specific speed of the ions. In this manner, depending on the respective value of the anode voltage, ionized particles with different specific masses can travel along the ion path and impinge on the ion catcher. By suitable selection of the anode voltage, one can determine the respective specific mass which is to be examined.
Mass spectrometers of the modulating type wherein offset and disturbance effects are suppressed by use of modulation technique are already known. These spectrometers modulate the disturbance effects in such a manner that the measurement signal is caused to change between a largest possible sensitivity and a smallest possible sensitivity. Thus, a large modulation depth is obtained, and disturbance effects can be optimally eliminated.
Various options for modulation are available, notably:                a. acceleration voltage (in the present case, anode voltage)        b. magnetic field        c. direction/site of ion entrance, by use of modulated deflection voltages        
As a test gas for leak detection devices, frequent use is made of helium. Helium has the specific mass M4. A difference resides in the restricted selectivity of the mass spectrometer. Due to this restriction, the signal peaks of the integral specific masses, which actually should be distinct from each other, are caused to merge into each other. For instance, one component of the M3 signal will spread into the range of the M4 signal so that, if a large quantity of an M3 gas exists, a gas of the specific mass M4 (e.g. helium) cannot be measured with sufficient selectivity. In practice, this is indeed the case. On containers for leak measurement, water will deposit both on the outside and on the inside of the container. The H2 component of water includes also M3 portions whose existence considerably disturbs the measurement of an M4 gas. Although one could perform the measurement under vacuum conditions long enough to allow the water components to be pumped off sufficiently, this approach would require a very long pumping period and thus cause a long time to pass until a stable indication of the leak rate is possible. The influence of the water on the measurement result is referred to as “water underground”.